1. Field of the Invention
The present invention generally relates to an apparatus for discriminating a condition of a storage tank for storing a liquid reducing agent suitable for use not exclusively but preferably in exhaust emission purification and in particular, to a technology for discriminating with high precision whether the storage tank is empty, a reducing agent is normally filled in the storage tank or the storage tank stores therein any dissimilar aqueous solution, by utilizing a concentration sensor which indirectly measures the concentration of the liquid reducing agent based on heat transfer characteristics between two positions close to each other.
2. Description of the Related Art
As a catalytic purification system for removing nitrogen oxides (NOx) contained in the exhaust emission of an engine, there has been proposed an exhaust emission purifying apparatus disclosed in Japanese Laid-open (Kokai) Patent Application Publication No. 2000-27627. In the exhaust emission purifying apparatus, a reducing agent according to engine operating conditions is injection-supplied to the upstream of the exhaust stream with respect to a reduction catalytic converter, which is disposed in an engine exhaust system, so that NOx in the exhaust emission and the reducing agent are subjected to the catalytic-reduction reaction to purify NOx into harmless components.
However, according to the above-described conventional exhaust emission purifying apparatus, when a purification efficiency is changed with a change in concentration of the liquid reducing agent, if a driver continues the operation of the engine without taking notice of this change, the likelihood that the required NOx purification performance is not successfully exhibited, resulting in an occurrence of an undesirable condition such that a large amount of discharge of NOx from the engine takes place. In particular, in the case where a storage tank is empty or the dissimilar aqueous solution which does not function as the liquid reducing agent is used, there is caused such an undesirable condition significantly.
Therefore, it may occur to a skilled person in the art to arrange a concentration sensor which indirectly measures the concentration of the liquid reducing agent based on the heat transfer characteristics between two positions close to each other. However, if such a concentration sensor is mounted on a movable vehicle such as an automobile, the following problems might be caused. Namely, during the driving of the movable vehicle, since a vehicle body is continually subjected to vibration due to the irregularity of road surfaces, the convection can be generated in the liquid reducing agent in the storage tank. Further, if the engine coolant or the like is circulated in the storage tank in order to prevent the freezing of the liquid reducing agent, the temperature distribution of the liquid reducing agent might become uneven, resulting in that, similar to the case of vehicle vibration, the convection can be generated in the liquid reducing agent in the storage tank. Then, if the convection is generated in the liquid reducing agent, the heat transfer characteristics using the liquid reducing agent as a heat transfer medium might be changed, and therefore, the measurement accuracy of the concentration of the liquid reducing agent might be significantly lowered.